Various countries in the world discharge sulfur dioxide to different extents. Sulfur dioxide emissions are huge in China, have a huge impact on the environment and society, and the total sulfur dioxide emission in 2014 was 19.74 million tons, and, in 2015, 18.591 million tons, ranking first in the world, resulting in a huge economic loss and a serious impact on ecological environment and people's health in China.
Currently there are hundreds of relatively mature desulfurization technologies, in which the wet desulfurization process is the most widely used, accounting for around 85% of the world's total installed capacity for desulfurization. Common wet flue gas desulfurization technologies are limestone-gypsum, double alkalis, sodium carbonate, ammonia, magnesium oxide methods, and the like. Ammonia desulfurization is a wet desulfurization process using ammonia as an absorbent, and this method can produce an ammonium sulfate fertilizer using SO2, and is a kind of green flue gas treatment scheme with low energy consumption, high added value and realized resource recycling. There is a large amount of ammonia wastewater generated during production in the chemical industry, and therefore using ammonia desulfurization for boiler exhaust gas in the chemical industry has its unique advantages.
Ammonia desulfurization process mainly includes three procedures, absorption, oxidation and concentration (crystallization), by:
Absorbing sulfur dioxide with ammonium sulfite to obtain a mixed solution of ammonium sulfite and ammonium bisulfite, to which ammonia is added to obtain ammonium sulfite:(NH4)2SO3+H2O+SO2NH4HSO3 (NH4)xH(2-x)SO3+(2-x)NH3═(NH4)2SO3 
Supplying oxidation air to the solution to oxidize ammonium sulfite to give ammonium sulfate:(NH4)2SO3+½O2═(NH4)2SO4; and
Subjecting the ammonium sulfate solution to concentration, crystallization, solid-liquid separation and drying, thereby obtaining the final product ammonium sulfate.
The three procedures, absorption, oxidation and concentration, seem simple. In fact, they influence each other. Conventionally, in order to ensure the absorption efficiency, ammonium sulfite and free ammonia contents were maintained at a high level, and the ammonium sulfate content was maintained at a low level in the absorption liquid, which is conducive to absorption, but not conducive to oxidation and concentration, and pH of the absorption liquid was maintained at about 7, thereby leading to serious ammonia escape and aerosols during absorption.
In order to ensure the absorption efficiency, conventionally, the absorption temperature was controlled to be not higher than 40° C. by cooling with process water, setting a reheater, lowering temperature with a dilute ammonium sulfate solution, and other measures that are conducive to absorption, but not conducive to oxidation and concentration. At a low temperature, ammonium sulfite at a high concentration cannot be completely directly oxidized to ammonium sulfate quickly, but same at a lower concentration can be subjected to oxidation and the evaporation and concentration processes to obtain a product, with a large amount of evaporation, large energy consumption, a long flow process, a lot of equipment, a large footprint, a high operation cost, and poor economical efficiency of the device. Furthermore, generally the water content of the boiler flue gas is maintained at not less than 7%. The water content of sulfur recovery exhaust gas, incineration flue gas and other industrial exhaust gas is even more than 25%. Therefore if the absorption efficiency is deliberately pursued by reducing the absorption temperature to not less than 40° C., not only is the energy consumption high, but also the water in the flue gas will condense. The condensed water is excess, is not conducive to rinsing the demister and rinsing the tower wall, and needs to be discharged in the form of waste water.
As for a dry method for the sulfuric acid exhaust gas, due to a low water content and a low concentration of sulfur dioxide, the absorption temperature can be controlled at 30° C.-50° C.
Ammonia desulfurization processes for flue gas may involve the following technical issues:
1. Ammonia Escape and Aerosols
Different from the limestone-gypsum method based on limestone as the raw material, the ammonia is easy to volatilize, when the free ammonia is present in the absorption liquid, ammonia, SO2 and SO3 are simultaneously present in the gas phase. Therefore, ammonium sulfite and ammonium sulfate mist is forms easily, and saturated water vapor in the flue gas condenses onto the mist by using the mist as a core, thereby forming dense white mist, which on the one hand causes ammonia loss, and on the other hand causes secondary pollution.
Until now the ammonia desulfurization failed to be effectively generalized, for which the main reason is that the previous efforts focus on how to capture aerosols produced during absorption, and not suppress or reduce aerosol production during absorption, resulting in a large system investment, high operation cost and unstable operation.
2. Oxidation of Ammonium Sulfite
The oxidation of ammonium sulfite is different from other sulfites, and NH4+ at a certain concentration has a damping effect on the oxidation process. Literature (for example, Zhou, J., W. Li, and W. Xiao, Kinetics Of Heterogeneous Oxidation Of Concentrated Ammonium Sulfite, Chemical Engineering Science, Volume 55, Issue 23, December 2000, Pages 5637-5641, Pergamon Press, Oxford, England, 2000, which is hereby incorporated herein by reference in its entirety) illustrates this unique property, i.e. NH4+ significantly blocks the dissolution of O2 in aqueous solutions. When the salt concentration is less than 0.5 mol/L (about 5% (wt)), the oxidation rate of ammonium sulfite increases with the increase of its concentration; and when this limit is exceeded, the oxidation rate decreases with the increase of the concentration. In addition, when the concentration of the total ammonium salt is 3-4 mol/L, and the concentration of ammonium sulfite is less than 0.15 mol/L, the oxidation reaction of the solution is a 0th-order rapid reaction, i.e., the oxidation rate is irrelevant to the ammonium sulfite content.
The oxidation reaction of ammonium sulfite actually also occur during absorption, but due to a low O2 content in the flue gas, a low temperature and a slow reaction speed, the oxidation rate is generally 40%-70% in continuous cycling conditions. However, further improving the oxidation rate to not less than 95% to meet the post-treatment processing requirements is still needed, so an oxidation tank/oxidation section/jet oxidizer has been used in to fully oxidize ammonium sulfite in a condition of excess and pressurized oxidation air, and some manufacturers choose to add a catalyst to the absorption liquid to promote the oxidation, but this will affect the product quality.
3. Recovery of the Exhaust Gas Entraining Ammonia
Different from other alkaline substances, ammonia is easily volatile. In traditional counter-current contact type absorption towers, either spray towers, packed towers or plate towers, in order to ensure the desulfurization efficiency and the final emission index, the pH value of a solution is the highest, the concentration of SO2 in the gas phase is the lowest and the concentration of ammonia in the gas phase will be the highest at a contact point at the top of the absorption zone. This means that the amount of ammonia spilling with the exhaust gas out of the desulfurization tower will be large. This will cause the waste and loss of ammonia, but also cause new pollution
As for the aerosols and ammonia escape problems, well-known research institutions and engineering companies have proposed a variety of schemes to control or eliminate, such as wet electricity, multi-stage water washing, multi-stage demisting or a combination thereof; however, these methods treat the problem not from the sources producing aerosols, and ammonia escape during absorption, only focusing on how to eliminate ammonia escape and aerosols produced during absorption, making the section number of towers more and more and the system more complex, which not only has poor treatment effects, but also has a substantial increase in investment and operation costs.
The absorption, oxidation and concentration of the ammonia desulfurization device interact with each other, the absorption requires a high pH value of the solution and a high ammonium sulfite content, the oxidation requires a relatively low total ammonium salt concentration and a low ammonium sulfite content, and the concentration requires a high ammonium sulfate content. Controlling ammonia escape and aerosols requires a low pH value and a solution not containing free ammonia.
Since the requirements of the solution compositions for different processes are different, more reasonable technologies for controlling aerosol production are highly required to achieve synergistic control of the absorption, oxidation and concentration, meet the emission requirements while reduce investment, simplify the technological process, and reduce the operation difficulty.
A Chinese patent for invention with an application number of CN 02136906.2 discloses a method and device for removal and recovery of SO2 in flue gas, in which the concentration of ammonium sulfite is controlled between 0.1%-5% (wt), for example, between 0.5% and 2.0%, to create favorable conditions for oxidation, reduce the energy consumption and investment of oxidation, and ensure a high desulfurization efficiency. The ratio of ammonia in the absorption liquid to sulfur is 1.3-1.8 (molar ratio), the ratio of the absorption gas to liquid is 2000-5000 (volume ratio). Hot flue gas heat is used for concentrating the ammonium sulfate solution, and when the hot flue gas temperature is reduced to 50° C.-55° C., the concentration of ammonium sulfate can be increased to 40%-50% (wt), which will be sent to an ammonium sulfate crystallizer and processed into a commercial ammonium sulfate fertilizer. The oxidation section is provided with a longitudinal partition, so that the unoxidized ammonium sulfite solution and oxidized ammonium sulfate solution are separated as far as possible, so as to prevent the occurrence of back-mixing. In this method: 1) the concentration of the absorption liquid is low, and the method is only suitable for low sulfur-containing flue gas; 2) the method does not concern about the control of ammonia escape and aerosol production during absorption, it is necessary to provide a reheater to eliminate white smoke; 3) the crystallization is influenced by the drying air volume and dust content, and the crystallization amount is small and not stable, and the like.
A Chinese patent for invention with an application number of CN 201310634675.7 discloses a desulfurization and denitrification system and a desulfurization and denitrification method therewith, wherein the absorption section includes an one-level circulation liquid spraying layer I, an one-level circulation liquid spraying layer II, a filler absorption layer and an one-level circulation liquid spraying layer III arranged from bottom to top sequentially, in which the one-level circulation liquid spraying layer I is a fixed ammonia absorption layer for efficiently absorbing SO2, the fixed ammonia absorption layer is a separate ammonia absorption circulation system, the one-level circulation liquid spraying layer II and the filler absorption layer are used for preventing ammonia escape and for absorbing SO2, and the one-level circulation liquid spraying layer III is used for preventing ammonia escape. However, the solution compositions are not specified, and the effect of controlling the ammonia escape and aerosols is limited by adding ammonia in layers.
A Chinese patent for invention with an application number of CN 201510009642.2 discloses a method for achieving integrated desulfurization and dust removal with ultrasonic waves for achieving ultra-low emissions, in which the flue gas which has been subjected to temperature reduction and desulfurization is fully washed by providing an absorption liquid droplet washing system, the droplet in the absorption liquid in flue gas is captured and removed, followed by demisting; after demisting, the flue gas is washed by the droplet of the absorption liquid, followed by demisting; the above-mentioned preliminarily purified flue gas is subjected to agglutination and/or coagulation so that the particle size of fine particles is enlarged, and the enlarged fine particles are removed by agglutination and/or a layer demister; and use of multi-stage water washing and multi-stage demisting ensures the total dust being qualified with large investment and high operation cost, which cannot control ammonia escape and aerosol production from the aspect of mechanism.
A Chinese patent for invention with an application number of CN 201510680578.0 discloses an ammonia double circulation desulfurization, denitrification and dust removal system, including a washing absorption tower (1) and an oxidation circulation tank (9); the washing absorption tower (1) consists of an efficient water mist removal section (2), an enhanced ammonia mist removal section (3), an absorption liquid demisting section (4), a secondary absorption section (5), a primary absorption section (6) and a washing and lowering temperature (7) sequentially; when the flue gas enters into the primary absorption section (6), SO2 is primarily removed by using an ammonium nitrate-containing ammonium sulfate solution with a density of 1.1 to 1.15 kg/L and a pH value of 6.5 to 7 as an absorption liquid; and when the flue gas enters into the secondary absorption section (5), SO2 is secondarily removed by using an ammonium nitrate-containing ammonium sulfate solution with a density of 1.05 to 1.1 kg/L and a pH value of 5.5 to 6 as an absorption liquid. The technological process is complex, with excess ammonia used during absorption, significant aerosol and ammonia escape, and final emission targets that are difficult guarantee by water washing and demisting.
A Chinese patent for invention with an application number of CN 201610390173.8 discloses an integrated desulfurization and dust removal device with a single tower having six gradient purification sections for achieving ultra-low emissions, including: an oxidation section, a concentration section, an absorption section, a purified water washing section, a demisting section, a partition and a wet electrical section; wherein the tiny droplets carried by demisted flue gas are further removed through the electrostatic adsorption action in the wet electrical section, so as to ensure that the discharge standard of flue gas is met when the working conditions for flue gas vary, and the section is used as an insurance measure for this device. The process requires large investment, has a high operation cost, and is an ineffective electrical method of controlling ammonia escape and aerosol emissions.
A Chinese patent for invention with an application number of CN 201610177178.2 discloses a combined desulfurization and dust removal process for achieving ultra-low emissions, a device therein includes a desulfurization tower (1); a flue gas inlet (2) and a flue gas outlet (9) are provided on the desulfurization tower (1); a washing and lowering temperature section (3), a primary absorption section (4), a secondary absorption section (5), a primary demisting section (6), a secondary demisting section (7), and a tertiary demisting section (8) are arranged in series in the flow direction of flue gas between the flue gas inlet (2) and the flue gas outlet (9); the flue gas containing SO2 of fuel coal at 120° C.-180° C. is subjected to denitrification and dust removal, then enters into the washing and lowering temperature section (3) from the flue gas inlet (2), and is sprayed with an ammonium sulfate solution with a density of 1200-1250 g/L and a pH value of 3-5, so as to reduce the temperature of the flue gas to be 45° C.-60° C.; the flue gas flows into the primary absorption section (4), and is sprayed with an absorption liquid with a density of 1100-1250 g/L and a pH value of 5.5-6.5; and when the flue gas enters into the secondary absorption section (5), it is sprayed with an absorption liquid with a pH value of 5.0-5.8 and a density of 1030-1100 g/L, and then the flue gas enters into the primary demisting section (6), the secondary demisting section (7) and the tertiary demisting section (8) sequentially, and is discharged from the flue gas outlet (9). The hot flue gas is subjected to gradient elution by controlling the density, pH value and the like of the absorption liquid, then subjected to mist droplet removal using a unique demister and other devices, and sulfur, smoke dust and the like in the flue gas can be effectively removed thereby achieving ultra-low emissions. However, the process does not specify the solution compositions and absorption temperatures, and still cannot control the ammonia escape and aerosol production from the source.
A Chinese patent for invention with an application number of CN 201611014433.8 discloses a method for reducing aerosol production in ammonia desulfurization, of which the particular steps are: 1) driving the aqueous ammonia into an ammonia absorption tower, initiating a primary absorption and circulation pump for spray washing, so as to desulfurize most SO2 in the flue gas; 2) driving the aqueous ammonia into the ammonia absorption tower for spray washing, where the spray liquid is further reacted with SO2 flue gas to remove pollutants in the flue gas; 3) passing the flue gas which has been subjected to the secondary absorption through a water washing and spraying device to wash impurities such as aerosols entrained in the flue gas; and 4) lastly purifying the flue gas, which has been subjected to water washing, against impurities such as liquid foams and residual aerosols entrained in the flue gas during washing and spraying, and discharging the cleaned exhaust gas up to standard. In step 1, the pH value of the absorption solution is strictly controlled at 5.5-6.5, and the density is controlled at 1.15-1.25 g/ml. In step 2, the pH value of the absorption solution is strictly controlled at 5.0-6.0, and the density is controlled at 1.0-1.20 g/ml. The process does not specify the solution compositions and absorption temperatures, and still cannot completely control the ammonia escape and aerosol production from the source. Also, the flue gas which has been subjected to simple water washing and demisting cannot meet or cannot easily meet the ultra-low emission standard requirements or higher requirements in China.
A Chinese patent for invention with an application number of CN201611207184.4 discloses a process for saving water and controlling the aerosol phenomenon in ammonia desulfurization process, wherein the boiler flue gas enters into a desulfurization tower, the SO2-containing flue gas which has entered into the desulfurization tower is sprayed with a spray liquid of ammonium sulfate/ammonium sulfite solution with a concentration of 5%-35%, followed by passing through a filler layer, contacting with the cooling water on the filler layer, and then contacting with a water washing and spraying layer, during which the cooling water at the bottom of the filler layer falls onto a water washing liquid accumulation pan and reversely flows to a cooling water tower, then enters into a water washing tank, and is driven to a water washing and spraying layer through a water washing water infusion pump for recycling; the system has advantages such as a simple flow process, a good cooling effect and a low operation cost, the spray cooling water absorbs substances such as (NH4)2SO4 particles, SO2, and NH3 in the boiler flue gas, the saturated water vapor in the boiler flue gas condenses, using the (NH4)2SO4 particles as the cores to form water droplets, so that the (NH4)2SO4 particles in the boiler flue gas are captured, thereby inhibiting the aerosol formation, and making the particle concentration in the boiler flue gas discharged in the ammonia desulfurization process be less than 30 mg/m3. The process does not specify the solution compositions, pH values and absorption temperatures, still cannot completely control ammonia escape and aerosol production from the source, furthermore, the energy consumption of low-temperature washing is high, and the particle concentration in purified flue gas is less than 30 mg/m3, which cannot meet the latest emission standards.
A Chinese patent for invention with an application number of CN 201310340885.5 discloses a method for controlling aerosol emission in ammonia desulfurization and a dedicated absorption tower therefor, wherein the flue gas which has been subjected to spraying with atomized water and lowering temperature and has been cooled to 100° C. to 120° C. is allowed to flow into a desulfurization zone of a desulfurization absorption tower, the flue gas in the desulfurization zone from bottom to top is allowed to contact with a desulfurization liquid countercurrent ejected from top to bottom to absorb SO2 in the flue gas, and fillers or sieve plates are provided within the desulfurization zone; the flue gas after been desulfurized enters into a filler washing zone, into which washing water in injected to remove coarse-grained aerosols produced in ammonia desulfurization; the flue gas after been subjected to desulfurization and coarse-grained aerosol removal enters into a water-vapor phase transition zone, steam is injected into the middle of the water-vapor phase transition zone so as to establish an oversaturate water vapor environment required for the water-vapor phase transition, so that unremoved fine-grained aerosol particulates condense and grow up and are removed by a wire mesh demister at a flue gas outlet of the water-vapor phase transition zone; and the purified flue gas is discharged via a chimney from the flue gas outlet at the top of the desulfurization absorption tower. The superficial gas velocity of the flue gas is 2.0-3.0 m/s and an operation liquid to air ratio is 2 to 8 L/Nm3; and the desulfurization liquid has a pH value of 5.2-6.0 and a temperature of 45° C.-55° C.; the desulfurizing agent in the desulfurization liquid is ammonium sulfate or ammonium sulfite at a concentration of 10% wt to supersaturated, the washing water spraying liquid-gas ratio in the filler washing zone is 0.6-3.0 L/Nm3, the temperature of the flue gas after been washed through the filler layer is reduced to 50° C.-55° C., and in an embodiment, at the absorption tower outlet, the minimal mass concentration of PM10 is 45 mg/m3 and the minimal SO2 concentration is 135 mg/Nm3. The process still cannot completely control ammonia escape and aerosol production from the source, furthermore, the particles and SO2 in the purified flue gas cannot meet the latest emission standards, and the energy consumption of vapor phase transition is high.
A Chinese patent for invention with an application number of CN 201610966033.0 discloses a device and method for removing aerosols in ammonia desulfurization, the device including a desulfurization tower (1), wherein the interior of the desulfurization tower (1) is provided with an absorption reaction zone (2), an oxidation water-washing zone (3) and a water washing and purifying zone (4) from bottom to top sequentially; an oxidation water-washing and spraying layer (22) is provided within the oxidation water-washing zone (3), and the concentration of ammonium sulfate dissolved in an oxidation water-washing circulation liquid is controlled at ≤3%; the temperature of water washing and purifying is ≤50° C.; and the strong oxidant includes hydrogen peroxide or hypochlorite. The process does not specify the solution compositions, pH values and absorption temperatures, and still cannot completely control the ammonia escape and aerosol production from the source, furthermore, the investment in oxidation water-washing is large, the operation cost is high, and there is a certain safety risk.